This invention relates generally to phase-frequency detection and more particularly to a digital phase-frequency detector.
Phase-locked loops are well known in the art and are electronic circuits for locking an oscillator in phase with an arbitrary input signal. The phase-locked loop is utilized in two fundamentally different ways; first as a demodulator, where it is employed to follow (and demodulate) frequency or phase modulation, and second to track a carrier or synchronizing signal which may vary in frequency with time.
An essential part of the phase-locked loop is the phase angle (.phi.) detector. Phase-locked loop performance is dependent on the loop's ability to control the frequency and phase of the voltage-controlled oscillator output. The typical phase detector adequately controls the output frequency and phase relationships when the loop is in the locked condition. However, when the loop is out of lock, i.e., the input reference signal and the loop output signal have a different frequency and phase relationship, acquisition and lock out of the input reference becomes a relatively slow process.
Under these conditions the output of this type of detector is in the form of one error beat frequency which slowly adjusts the output frequency in the proper direction. A more desirable phase detector output, under these conditions, would be a voltage which would move the output frequency in the proper direction at an increased rate.
A currently available phase-frequency detector that will correct a frequency difference more quickly than other known detectors is the Motorola MC-12040 (R) phase-frequency detector. The disadvantage of this and similar detectors is a condition which occurs when the phase locked loop is locked and no phase error exists. This design of phase detector exhibits a "dead zone" in which the phase of the loop's output is not tightly controlled. The "dead zone" is an undesirable characteristic because a lack of phase control will, with substantial probability, result in phase jitter, an undesirable characteristic well known in the art.
The phase-frequency detector described herein exhibits rapid acquisition with accurate control of phase thus eliminating any "dead zone" and the possibility of phase jitter.